Dimmable electronic ballast for electrodeless discharge lamp and luminaire

ABSTRACT

A dimmable electronic ballast for an electrodeless discharge lamp comprises an inverter circuit, a resonance circuit, an induction coil and a start circuit. The start circuit has a variable time constant. The start circuit sweeps a drive frequency of the inverter circuit through a time constant for start or restart so that the voltage applied across the coil is raised from voltage lower than start voltage and restart voltage for starting and restarting the lamp to voltage higher than the start voltage and the restart voltage. The time constant for start during a start period for starting the lamp is larger than the time constant for restart during a restart period for restarting the lamp.

TECHNICAL FIELD

The invention relates to a dimmable electronic ballast for anelectrodeless discharge lamp and luminaire equipped with the ballast.

BACKGROUND ART

Dimming in a dimmable electronic ballast for an electrodeless dischargelamp is realized by the repetition of turning the lamp on and off in thesame way as, for example, the circuit described in Japanese PatentApplication Publication Number 2000-353600. In such a circuit, each oflamp turn on and off periods is set to a short period of time (e.g., afew ms or less) in which brightness change by the lamp is not perceivedby the human eyes. In the circuit, each lamp turn off period is set toapproximately 0.5 ms.

This sort of ballast is made up of, for example, a DC power supplycircuit, an inverter circuit, a resonance circuit and an induction coil.In addition, the ballast is further provided with a start circuit forsweeping up the start voltage applied across the coil in order to startan electrodeless discharge lamp successfully and stably.

In that sweep-up type ballast, the start voltage swept up during a startperiod becomes very high owing to the time constant of the startcircuit. Because restart voltage must be raised up to voltage requiredto restart the lamp immediately after a short period of lamp turn off,and also the start period is longer than the restart period.

When the ballast is further provided with a ferrite core like, e.g., thesystem described in Japanese Patent National Publication NumberP2003-515898A (WO01/041515), if ambient temperature is high, it easilysaturates the ferrite core during a start period.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to lower startvoltage without lowering restart voltage, and also, even if a ferritecore is installed, to hardly saturate the ferrite core during a startperiod.

A dimmable electronic ballast for an electrodeless discharge lamp of thepresent invention comprises an inverter circuit, a resonance circuit, aninduction coil and a start circuit. In response to a drive signal havinga variable drive frequency the inverter circuit converts the DC powerfrom a DC power supply circuit into the high-frequency power having theoperation frequency corresponding to the drive frequency. The resonancecircuit receives the high-frequency power and then produceshigh-frequency resonant power based on a resonance characteristic. Theresonant power is variable output corresponding to the operationfrequency. The induction coil receives the high-frequency resonant powerand then generates a high-frequency electromagnetic field to apply theelectromagnetic field to the electrodeless discharge lamp. The startcircuit has a variable time constant. The start circuit sweeps the drivefrequency through a time constant for start or restart so that thevoltage applied across the coil by the high-frequency resonant power israised from voltage lower than start voltage and restart voltage forstarting and restarting the lamp to voltage higher than the startvoltage and the restart voltage. The time constant for start during astart period for starting the lamp at the beginning of dimming is largerthan the time constant for restart during a restart period forrestarting the lamp in the dimming period.

In this construction, during each restart period, the voltage appliedacross the coil can be quickly raised through the time constant forrestart, and therefore the lamp can be quickly restarted and then turnedon. Moreover, it is possible to reduce stress on circuits, because therise of the voltage applied the coil during the start period can be madegentle through the time constant for start and also the voltage (maximumvoltage) at the time of start can be decreased. Therefore, the startvoltage can be lowered without lowering the restart voltage. Inaddition, even if a ferrite core is installed, the ferrite core can behardly saturated during a start period.

In a preferred embodiment, the start circuit receives a dimming controlsignal that is variable in duty and repeats first and second levels. Inresponse to each first level, the start circuit periodically sweeps thedrive frequency through the time constant for restart so that thevoltage applied across the coil is raised from the voltage lower thanthe restart voltage to the voltage higher than the restart voltage. Thelamp is repeatedly turned on and off in accordance with the dimmingcontrol signal, and is lit at the dimming rate corresponding to theduty.

In another preferred embodiment, the start circuit periodically changesthe drive frequency to a frequency for extinction during each secondlevel of the dimming control signal. The frequency for extinction is afrequency for lowering the voltage applied across the coil to voltagelower than voltage necessary to light the lamp. However, not limited tothis, the inverter circuit may periodically stop the output of itselfduring each second level of the dimming control signal.

In an enhanced embodiment, the start circuit continuously sweeps downthe time constant for start and the time constant for restart. In thisembodiment, the components of overshoot and undershoot are excluded andaccordingly the restart voltage can be prevented from becoming excessiveowing to such components.

In another enhanced embodiment, the start circuit is connected with acontrol signal generation device. The control signal generation devicesupplies the dimming control signal to the start circuit, and alsoincreases duty of each second level of the dimming control signal fromthe output time point of the dimming control signal. The duty isincreased from zero to a given value. In case of this embodiment, theovervoltage generation caused by change of the time constant can beprevented.

In other enhanced embodiment, the start circuit is connected with acontrol signal generation device. The control signal generation devicesupplies the dimming control signal to the start circuit before a timeof 50 ms passes from a point in time at which the resonancecharacteristic of the lamp shifts to the resonance of a lighting mode.In case of this embodiment, a flicker feel after a start period can besuppressed.

In other preferred embodiment, a first operation frequency immediatelybefore sweeping in the start period is lower than a second operationfrequency immediately before sweeping in the restart period. The voltageapplied across the coil by the first operation frequency is also higherthan the voltage applied across the coil by the second operationfrequency. In case of this embodiment, the start voltage can be furtherdecreased and also a start period can be shortened. Furthermore, thesaturation of a ferrite core can be more effectively prevented.

Luminaire of the present invention comprises said dimmable electronicballast, and is equipped with the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in furtherdetails. Other features and advantages of the present invention willbecome better understood with regard to the following detaileddescription and accompanying drawings where:

FIG. 1 is a circuit diagram of a first embodiment of a dimmableelectronic ballast for an electrodeless discharge lamp according to thepresent invention;

FIG. 2 illustrates luminaire equipped with the ballast of FIG. 1;

FIG. 3 shows another example of luminaire equipped with the ballast ofFIG. 1;

FIG. 4 is a circuit diagram of a drive circuit in the ballast of FIG. 1;

FIG. 5 is an input/output characteristic curve of the drive circuit ofFIG. 4;

FIG. 6 is a resonant characteristic curve of the ballast of FIG. 1;

FIG. 7 shows an operational principle of a time constant change circuitin the ballast of FIG. 1;

FIG. 8 is a timing diagram showing the operation of the ballast of FIG.1;

FIG. 9 is a circuit diagram of a control signal generation device and astart circuit in a second embodiment of a dimmable electronic ballastfor an electrodeless discharge lamp according to the present invention;

FIG. 10 shows an alternate embodiment;

FIG. 11 is a timing diagram showing the operation of the embodiment ofFIG. 10;

FIG. 12 shows a variable duty on HIGH period of a dimming control signalin a third embodiment of a dimmable electronic ballast for anelectrodeless discharge lamp according to the present invention;

FIG. 13 is a timing diagram showing the operation of the ballast of FIG.12;

FIG. 14 shows output waves of a fourth embodiment of a dimmableelectronic ballast for an electrodeless discharge lamp according to thepresent invention;

FIG. 15 is a circuit diagram of a start circuit in a fifth embodiment ofa dimmable electronic ballast for an electrodeless discharge lampaccording to the present invention; and

FIG. 16 is a timing diagram showing the operation of the ballast of FIG.15.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a first embodiment of a dimmable electronic ballast for anelectrodeless discharge lamp according to the present invention. Theballast in the first embodiment is installed to luminaire 1 such as, forexample, a street lamp 1A of FIG. 2, a protective lamp 1B of FIG. 3, adownlight or the like. The luminaire 1 is equipped with an electrodelessdischarge lamp 2. The lamp 2 has a bulb such as a glass bulb, a glasssphere or the like, filled with a discharge gas such as inert gas, metalvapor and so on (e.g., mercury and rare gas). The bulb is transparent orincludes phosphor applied to the inner surface.

As shown in FIG. 1, the ballast is formed of a DC power supply circuit11, an inverter circuit 12, a resonance circuit 13, an induction coil14, a control signal generation device 15 and a start circuit (controlcircuit) 16. However, the device 15 may be an external device.

The DC power supply circuit 11 is, for example, a voltage step upconverter and converts the AC power from an AC power source 3 into DCpower, i.e., DC voltage VDC. The converter is formed of, for example, arectifier (diode bridge) 110, an inductor 111, an FET 112, a diode 113,a smooth capacitor 114 and a control circuit 115.

The inverter circuit 12 is, for example, a half bridge inverter. Inresponse to a drive signal having a variable drive frequency theinverter converts the DC power from the DC power supply circuit 11 intothe high-frequency power having the operation frequency (e.g., tens kHzto tens MHz) corresponding to the drive frequency. The inverter is madeup of FETs 120 and 121, and a drive circuit 122.

As shown in an example of FIG. 4, the drive circuit 122 is formed of avoltage controlled oscillator (VCO) 123, resistors 124-126, a constantvoltage source 127, a diode 128 and a follower amp 129. The drivecircuit supplies each gate of the FETs 120 and 121 with the drive signalsuch as square waves or the like. Both of the drive signals have thephase difference of approximately 180°, and are applied across terminalsH_(OUT) and H_(GND) and terminals L_(OUT) and L_(GND). Accordingly, FETs120 and 121 are alternately turned on and off through the drive signals.

The drive circuit 122 also controls each drive frequency of the drivesignals in accordance with a drive control signal (voltage) V_(swp) fromthe start circuit 16. A basic frequency of the drive frequency isdetermined by the constant voltage source 127 and the resistors 124 and125. The basic component of input voltage V_(in) to the VCO 123 is thevoltage (divided voltage) obtained by dividing the voltage of theconstant voltage source 127 by the resistors 124 and 125. The (electric)current I_(swp) drawn from the division (coupling) point of theresistors 124 and 125 decreases and increases in response to increaseand decrease of the signal V_(swp), respectively. The voltage V_(in)increases and decreases in response to decrease and increase of thecurrent I_(swp), respectively. The VCO 123 then decreases and increaseseach drive frequency f_(dr) of the drive signals in response to theincrease and decrease of the voltage V_(in), respectively. Therefore, asshown in FIG. 5, the circuit 122 decreases and increases each drivefrequency f_(dr), of the drive signals in accordance with the increaseand decrease of the drive control signal V_(swp) respectively.

The resonance circuit 13 of FIG. 1 is, for example, a series resonancecircuit formed of an inductor 130 and a capacitor 131. The resonancecircuit receives the high-frequency power from the inverter circuit 12and then produces high-frequency resonant power based on a resonancecharacteristic. The resonant power is variable output corresponding tothe operation frequency of the circuit 12. As shown in FIG. 6, theresonance characteristic shifts to a first resonance curve SP and asecond resonance curve LP in response to the extinguishing and lightingstates of the lamp 2, respectively. The curve SP is the characteristicof a start period (a start mode) before the lamp 2 is lit, and has aresonant peak at a particular resonant frequency. The curve LP is thecharacteristic of a lighting period (a lighting mode) while the lamp 2is lit, and is lower than the first resonance curve SP. The capacitor132 of FIG. 1 is combined with the capacitor 131 to constitute amatching circuit.

The induction coil 14 is connected to the output of the resonancecircuit 13 and also located in the proximity of the lamp 2. The coil 14receives the high-frequency resonant power from the circuit 13 and thengenerates a high-frequency electromagnetic field to apply theelectromagnetic field to the lamp 2. The coil 14 is also provided with aferrite core.

The control signal generation device 15 supplies the start circuit 16with a composite control signal V_(pwm) including start and dimmingcontrol signals, and also supplies the circuit 16 with a time constantchange signal V_(t). The signal V_(t) is turned HIGH during a givenperiod from a start time point, and is turned LOW after the period.Adjustment of a dimming rate is realized by controlling ON duty of thedimming control signal included in the composite control signal V_(pwm).

The start circuit 16 is a sweep circuit having a variable time constant.As shown in FIGS. 5 and 6, the circuit 16 sweeps down the drivefrequency f_(op) through a time constant for start or restart so thatthe voltage V_(out) applied across the coil 14 by the high-frequencyresonant power is raised from voltage lower than start voltage andrestart voltage for starting and restarting the lamp 2 (see V_(out1) inFIG. 8) to voltage higher than the start voltage and restart voltage.

As shown in FIG. 1, the start circuit 16 is formed of a capacitor 160,resistors 161-165, an OP amp 166, a FET 167 and a time constant changecircuit 168. The capacitor 160 and the resistor 161 constitute anintegral circuit having a basic time constant, and are applied with DCvoltage V_(E). The OP amp 166 and the resistors 162 and 163 constitute anon-inverting amplifier, and amplify the output of the integral circuit.The FET 167 and the resistor 164 (resistor 164<resistor 165) constitutea discharge circuit. The discharge circuit discharges an electric chargeof the capacitor 160 during turn-on of the FET 167 at each HIGH of thecomposite control signal V_(pwm). The resistors 161, 164 and 165determine the minimum output voltage (minimum voltage of V_(swp)) of thenon-inverting amplifier when FET 167 is turned on, and then defines asweep start frequency of the drive frequency f_(dr). Accordingly, themaximum operation frequency f_(op1) at the time of start and extinctionis defined as shown in FIG. 6. The frequency f_(op1) is set to a valuethat losses in the inverter circuit 12, the coil 14 and so on becomefew. On the other hand, the maximum voltage across the capacitor 160determines the maximum output voltage of the non-inverting amplifier todefine the sweep end frequency of the frequency f_(dr). Accordingly, theminimum operation frequency f_(op4) is defined, and the lamp 2 is litwhen the operation frequency f_(op) is sweeping down from f_(op1) tof_(op4). In FIG. 6, f_(op3) is the operation frequency when the lamp 2is lit in the start period, and f_(op2) is the operation frequency whenthe lamp 2 is lit in the restart period.

As shown in FIG. 7, the time constant change circuit 168 changes a timeconstant τ of the start circuit 16 to the time constant for start (τ0)during the start period for starting the lamp 2. The circuit 168 changesthe time constant τ to the time constant for restart (τ1) during therestart period for restarting the lamp 2. Because of this, for exampleas shown in FIG. 1, the circuit 168 is formed of a transistor 168 a,capacitor 168 b and a resistor 168 c. The circuit 168 changes the timeconstant τ to τ0 when the time constant change signal V_(t) is HIGH, andchanges the time constant τ to τ1 when V_(t) is LOW. The τ1 iscalculated by C160×R161 and is set to a value of several ms. The C160 isthe capacitance of the capacitor 160, and R161 is the resistance of theresistor 161. The τ0 is calculated by (C160+C168 b)×R161 and is set to avalue within a given range larger than τ1. The C168 b is the capacitanceof the capacitor 168 b.

The operation of the first embodiment is now explained with reference toFIG. 8. At the start time point t10, the time constant change signalV_(t) of HIGH is supplied to the start circuit 16, and the time constantτ is set to τ0. The composite control signal (start control signal)V_(pwm) of HIGH is also supplied to the circuit 16, and the drivecontrol signal V_(swp) of the circuit 16 is set to the minimum voltagecorresponding to the sweep start frequency of the drive frequencyf_(dr). Accordingly, the drive signal having the sweep start frequencyis supplied from the drive circuit 122 to each gate of the FETs 120 and121, and the inverter circuit 12 operates at the maximum operationfrequency f_(op1).

Subsequently, at and after the time point t11 at which the compositecontrol signal (start control signal) V_(pwm) shifts from HIGH to LOW,the voltage across the capacitor 160 sweeps up through the time constantτ0 and the drive control signal V_(swp) sweeps up from the minimumvoltage. Thereby the drive frequency f_(dr) sweeps down and then theoperation frequency of the inverter circuit 12 sweeps down from f_(op1)to f_(op4), and therefore the voltage V_(out) applied across the coil 14rises. The lamp 2 is lit at the voltage V_(out3) before a time point t12at which the operation frequency reaches f_(op4). At this point, ahigh-frequency plasma current is generated inside the lamp 2 through thehigh-frequency electromagnetic field from the coil 14, and the lamp 2emits ultraviolet rays or visible light.

Subsequently, at and after the time point t13 at which the compositecontrol signal (dimming control signal) V_(pwm) shifts from LOW to HIGH,the drive control signal V_(swp) is returned to the minimum voltage. Thecomposite control signal (dimming control signal) V_(pwm) is also set toa constant value from 100 Hz to several kHz. Accordingly, the invertercircuit 12 again operates at the maximum operation frequency f_(op1) andthe lamp 2 is turned off. At the time point t14, the time constantchange signal V_(t) shifts from HIGH to LOW and the time constant τ ischanged to τ1.

Subsequently, at and after the time point t15 at which the compositecontrol signal (dimming control signal) V_(pwm) shifts from HIGH to LOW,the voltage across the capacitor 160 sweeps up through the time constantτ1 and the drive control signal V_(swp) sweeps up from the minimumvoltage. Thereby the drive frequency f_(dr) sweeps down and then theoperation frequency of the inverter circuit 12 sweeps down from f_(op1)to f_(op4), and therefore the voltage V_(out) rises. The lamp 2 is turnon through the voltage V_(out2) before the time point t16 at which theoperation frequency reaches f_(op4). Herein, the discharge gas withinthe lamp when it is restarted maintains energy and therefore the restartvoltage V_(out2) becomes lower than the start voltage V_(out3).

Subsequently, at and after the time point t17 at which the signalV_(pwm) shifts from LOW to HIGH, the drive control signal V_(swp) isreturned to the minimum voltage. Accordingly, the inverter circuit 12again operates at the maximum operation frequency f_(op1) and the lamp 2is turned off. At and after this operation, the same operation isrepeated.

In the first embodiment, during each restart period in the dimming mode,the voltage V_(out) can be quickly raised through the time constant forrestart τ1, and therefore the lamp 2 can be quickly restarted and thenturned on in response to LOW of the signal V_(pwm).

Moreover, stress on circuits can be reduced (see FIG. 7), because therise of the voltage V_(out) during the start period can be made gentlethrough the time constant for start τ0 and also the start voltage(maximum voltage) V_(out) can be decreased. This advantage was confirmedby an experiment. When the time constant τ at the time of start was 0.4ms and was the same as the time constant at the time of restart, themaximum voltage at the time of start was 1.65 kVo-p under the conditionthat the frequency f_(pwm) of the voltage (pulse width modulationvoltage) V_(pwm) was 500 Hz and the operation frequency f_(op3) at thetime of start was 135 kHz. When the time constant for start τ0 was 35ms, the maximum voltage at the time of start was 1.15 kVo-p under thecondition. Therefore, the start voltage can be lowered without loweringthe restart voltage. In addition, even if a ferrite core is installed,the ferrite core can be hardly saturated during a start period.

In an alternate embodiment, the inverter circuit periodically stops theoutput of itself during each HIGH of the composite control signalV_(pwm) (minimum voltage of V_(swp)) instead of decreasing the voltageV_(out) to V_(out1) (cf. FIG. 8).

FIG. 9 shows a control signal generation device 25 and a start circuit26 in a second embodiment of a dimmable electronic ballast for anelectrodeless discharge lamp according to the present invention. Theballast in the second embodiment is characterized by the start circuit26. The circuit 26 comprises a capacitor 260, resistors 261-265, an OPamp 266 and a FET 267 like those of the first embodiment, and alsocomprises a time constant change circuit 268 different from that of thefirst embodiment.

The time constant change circuit 268 is formed of a transistor 268 a andresistors 268 b and 268 c. The circuit 268 sets the time constant τ toτ0 when the time constant change signal V_(t) is HIGH, and changes thetime constant τ to τ1 when V_(t) is LOW. Under the condition that thecurrent between the emitter and base of the transistor 268 a isignorable small, τ1 is calculated by C260×(R261//R268 b) and set to avalue of several ms. The C260 is the capacitance of the capacitor 260,and R261 and R268 b are the resistances of the resistors 261 and 268 b,respectively. The τ0 is calculated by C260×R261 and set to a valuewithin a given range larger than τ1.

In an alternate embodiment, as shown in FIGS. 10 and 11, the startcircuit 26 continuously sweeps down the time constant for start τ0 andthe time constant for restart τ1 each in response to the time constantchange signal V_(t) from the control signal generation device 25. Thedevice 25 sweeps down the signal V_(t) from the sweep start time pointt21 of the start period to, for example, the time point t27. In case ofthis embodiment, the voltage applied to the non-inverting input terminalof the OP amp 266 does not include the components of the overshoot andundershoot like the second embodiment. Accordingly, the restart voltagecan be prevented from becoming excessive owing to such components.

FIGS. 12 and 13 show the operation of a third embodiment of a dimmableelectronic ballast for an electrodeless discharge lamp according to thepresent invention. The ballast in the third embodiment is characterizedby a control signal generation device. The control signal generationdevice increases duty of each HIGH of the dimming control signalincluded in the composite control signal V_(pwm) from the output timepoint t33 of the dimming control signal. The duty is increased from zeroto a given value at a time point t34. As the duty of HIGH is smaller, arate of ionized ions remaining in the lamp when it is restarted becomeslarger and therefore the maximum voltage at the time of restartgradually rises. The voltage applied to the non-inverting input terminalof the OP amp in the start circuit has the tendency that becomesinstable when the time constant is changed. However, the maximum voltageat the time of restart is lowered, and thereby the overvoltage caused bychange of the time constant can be suppressed.

FIG. 14 shows the output waves of a fourth embodiment of a dimmableelectronic ballast for an electrodeless discharge lamp according to thepresent invention. The ballast in the fourth embodiment is characterizedby a control signal generation device. The control signal generationdevice supplies the start circuit with the dimming control signal beforea time of 50 ms passes from the time point at which the resonancecharacteristic of the lamp shifts to the resonance of the lighting mode(second resonance curve LP in FIG. 6). In an example of FIG. 14, thedimming control signal is provided at the time point that 42 ms passesfrom the time point at which the resonance characteristic shifts to thesecond resonance curve LP.

A sense of a person who receives light stimulus in the eyes reaches thepeak after 50-100 ms from receiving the light stimulus. This phenomenonis known as the Broca-Sulzer effect. In the fourth embodiment, by makingΔT of FIG. 14 shorter than 50 ms, a flicker feel after a start periodcan be suppressed.

FIG. 15 shows a start circuit 56 in a fifth embodiment of a dimmableelectronic ballast for an electrodeless discharge lamp according to thepresent invention. The circuit 56 comprises a capacitor 560, resistors561-565, an OP amp 566, a FET 567 and a time constant change circuit 568like those of the first embodiment, and also further comprises a startvoltage change circuit 569.

In response to the time constant change signal V_(t) from the controlsignal generation device, the start voltage change circuit 569 causesthe operation frequency f_(op1′) immediately before sweeping in thestart period to be lower than the operation frequency f_(op1)immediately before sweeping in a restart period. Thereby, the circuit569 causes the voltage V_(out1′) applied across the coil by thefrequency f_(op1′) to be higher than the voltage V_(out1) applied acrossthe coil by the frequency f_(op1). The voltage V_(out1′) is lower thanthe level when the lamp shifts to arc discharge, and is set to a levelfor increasing a rate of ionized ions of the discharge gas in the lamp.However, the V_(out1′) may be a level when glow discharge is generatedin the lamp.

Accordingly, as shown in an example of FIG. 15, the start voltage changecircuit 569 is formed of a transistor 569 a and resistors 569 b and 569c. The resistance of a parallel circuit (564//565) formed of theresistors 564 and 565 is also set to a value larger than that of164//165 in the first embodiment. Thereby when the transistor 569 a isturned off in response to the time constant change signal V_(t) of HIGHat the start point t50, the voltage V_(swp) becomes higher than that ofthe first embodiment and the operation frequency f_(op) is set tof_(op1′). Consequently, the voltage applied across the coil during theperiod t50-t51 becomes voltage V_(out1′) higher than the V_(out1) of thefirst embodiment. The resistance of a parallel circuit of 564//565 andthe resistor 569 b is also set to almost the same value as that of164//165. Accordingly, when the transistor 569 a is turned on inresponse to the time constant change signal V_(t) of LOW at the timepoint t53, the voltage V_(swp) becomes low and an operation frequencyf_(op) at the time of turn-off is changed to f_(op1) similar to thefirst embodiment from the f_(op1′).

In the fifth embodiment, the voltage V_(out) immediately before sweepingdown in a start period is raised from V_(out1) to V_(out1′), and therebycapable of storing more energy in the discharge gas in the lamp.Accordingly, the start voltage V_(out3) can be decreased lower than thatof the first embodiment and also a start period can be shortened.Furthermore, the saturation of the ferrite core can be more effectivelyprevented.

Although the present invention has been described with reference tocertain preferred embodiments, numerous modifications and variations canbe made by those skilled in the art without departing from the truespirit and scope of this invention.

1. A dimmable electronic ballast for an electrodeless discharge lamp,comprising: an inverter circuit that, in response to a drive signalhaving a variable drive frequency, converts the DC power from a DC powersupply circuit into the high-frequency power having the operationfrequency corresponding to the drive frequency; a resonance circuit thatreceives the high-frequency power and then produces high-frequencyresonant power based on a resonance characteristic, said resonant powerbeing variable output corresponding to the operation frequency; aninduction coil that receives the high-frequency resonant power and thengenerates a high-frequency electromagnetic field to apply theelectromagnetic field to the electrodeless discharge lamp; and a startcircuit having a variable time constant, said start circuit sweeping thedrive frequency through a time constant for start or restart so that thevoltage applied across the coil by the high-frequency resonant power israised from voltage lower than start voltage and restart voltage forstarting and restarting the lamp to voltage higher than the startvoltage and the restart voltage; wherein the time constant for startduring a start period for starting the lamp at the beginning of dimmingis larger than the time constant for restart during a restart period forrestarting the lamp in the dimming period.
 2. The dimmable electronicballast of claim 1, wherein: the start circuit: receives a dimmingcontrol signal that is variable in duty and repeats first and secondlevels; and, in response to each first level, periodically sweeps thedrive frequency through the time constant for restart so that thevoltage applied across the coil is raised from the voltage lower thanthe restart voltage to the voltage higher than the restart voltage; andthe lamp is repeatedly turned on and off in accordance with the dimmingcontrol signal, and is lit at the dimming rate corresponding to theduty.
 3. The dimmable electronic ballast of claim 2, wherein the startcircuit periodically changes the drive frequency to a frequency forextinction during each second level of the dimming control signal, saidfrequency for extinction being a frequency for lowering the voltageapplied across the coil to voltage lower than voltage necessary to lightthe lamp.
 4. The dimmable electronic ballast of claim 2, wherein theinverter circuit periodically stops the output of itself during eachsecond level of the dimming control signal.
 5. The dimmable electronicballast of claim 2, wherein the start circuit continuously sweeps downthe time constant for start and the time constant for restart.
 6. Thedimmable electronic ballast of claim 2, wherein the start circuit isconnected with a control signal generation device, said control signalgeneration device supplying the dimming control signal to the startcircuit, and also increasing duty of each second level of the dimmingcontrol signal from the output time point of the dimming control signal,said duty being increased from zero to a given value.
 7. The dimmableelectronic ballast of claim 2, wherein the start circuit is connectedwith a control signal generation device, said control signal generationdevice supplying the dimming control signal to the start circuit beforea time of 50 ms passes from a point in time at which the resonancecharacteristic of the lamp shifts to the resonance of a lighting mode.8. The dimmable electronic ballast of claim 2, wherein: a firstoperation frequency immediately before sweeping in the start period islower than a second operation frequency immediately before sweeping inthe restart period; and the voltage applied across the coil by the firstoperation frequency is higher than the voltage applied across the coilby the second operation frequency.
 9. A luminaire comprising thedimmable electronic ballast of claim 2, wherein the luminaire isequipped with said lamp.